Radio communication over large or wide geographical areas has posed significant problems in the past. Systems of this type generally must make communication possible between mobile or portable equipment which have low power transmitters and which can move in any direction within the wide area coverage. There are several types of fixed system configurations which have been used to make such communication possible. One general type of system includes a centralized fixed-site receiver, a high power transmitter and a plurality of remotely located receivers connected by wireline or microwave back to the central site. Another type includes mobile repeaters which rebroadcast the radio communications, and a third type of system simultaneously transmits the radio communication or message from multiple fixed sites on the same frequency. Systems of this last type have often been referred to as "simulcast" systems.
Although many kinds of these systems have been successfully implemented to cover substantially wide geographical areas, none of these approaches are handily designed to cover inaccessible areas, or frequency blocked areas. These areas include subterraneous locations such as subways and sub-basements. Systems employing high power tranmitters or mobile repeaters cannot overcome the frequency blocking problem since reception of the frequency is inhibited. Simulcast systems, however, have enjoyed some success in overcoming this problem.
Simulcast systems generally include a plurality of radio transmission and reception fixed sites, spaced apart throughout the geographical area to be provided with radio communication coverage. Ideally, the fixed sites are distributed such that areas not covered by one site will be covered by another site. The sites are consequently distributed to create an overlapping patchwork of individual radio coverage areas which, taken together, approximates the desired geographical coverage area. The message to be transmitted from each site is conveyed from one site to each of the other sites by microwave or wireline and subsequently transmitted by all of the sites on the same frequency to be received by all of the mobiles and other portables within the wide coverage area.
Employing a simulcast system to include coverage for otherwise inaccessible areas is impractical since it generally entails installation of one or more additional radio transmission and reception fixed sites within the inaccessible area.
Notwithstanding the burdensome cost of the additionally installed microwave communication equipment, a number of problems are encountered with simulcast systems. One problem pertains to overlapping coverage areas necessary in simulcast systems to assure complete coverage by the transmitters. Because the transmitters operate on the same frequency, beat note interference and demodulated audio phase cancellation can result in the overlapping areas. Correction of these problems is expensive. Although it is not necessary to implement multiple transmitters within the inaccessible area, this problem remains inherent to the system if the system is to provide normal wide area coverage. The present invention avoids these problems by utilizing a novel technique operative within a trunked access system.
Trunked access radio systems are well known. In such systems, a fixed number of duplex radio channels is shared amoung a large number of potential users. Through a central controller, the system allocates the radio channels to the mobile users depending upon their communication need and deallocates the radio channel upon completion of a user's transmission. Communication between users and the central controller occurs on a designated link which can be a separate duplex control channel or some form of subaudible, audible, supra-audible or subcarrier shared radio channel communication.